Optically pure cyanohydrins are versatile synthetic intermediates, the two functional groups being easily manipulated into a wide range of other chiral products such as .alpha.-hydroxy acids, .alpha.-hydroxy aldehydes, .alpha.-hydroxy ketones, .beta.-hydroxy amines and .alpha.-amino acid derivatives etc.
The usual synthetic route to cyanohydrins was invented more than one hundred years ago. However, the cyanohydrin produced by this well known way is a racemic product. If optically active cyanohydrins are required for further asymmetric synthesis, additional resolution steps for the racemic cyanohydrin would be needed. To overcome this problem, several asymmetrical synthesis of cyanohydrin have been developed. Most of these asymmetric syntheses used a chiral catalyst to induce the formation of just one enantiomer of the cyanohydrin. So far, a number of different catalyses have been investigated, including enzymes, polymeric reagents, peptides and organometallic species. Among them, organometallic species are the catalysts developed most recently.
Organometallic species, which were used as the catalyses to catalyze the asymmetric addition of trimethylsilyl cyanide to aldehydes, were disclosed in several technical literatures. However, drawbacks could also be found among these organometallic species. For example, the organometallic complexes of binaphthaol and titanium tetraisopropoxide, reported by Nakai et al., have enantioselectivity only for aromatic aldehydes. The organometallic complexes reported by Jiang et al. have enantioselectivity but are easily deteriorated by acids. Bolm et al. also reported organometallic complexes with good enantioselectivity, but stoichiometric amounts of the organometallic complexes catalyst were required.
The inventor Uang et al. disclosed new ligands, i.e. ligand(I) and ligand (I'), ##STR2##
that can complex with titanium tetraisopropxide [Ti(O.sup.i Pr).sub.4 ] to form catalysts of the addition of trimethylsilyl cyanide to benzaldehyde in the pending patent application (application no. TW 87112176). The organometallic complexes catalyst containing chiral ligand (I) has acceptable enantioselectivities for all kinds of aldehydes. In particular, high enantioselectivities can be observed for aliphatic aldehydes. As used for catalyzing the synthesis of the asymmetric addition of trimethylsilyl cyanide to aldehydes to give optically active cyanohydrins, only catalytic amounts of the chiral ligand (I) (or (I')) are required. Although acceptable enantioselectivities for all kinds of aldehydes can be observed when the chiral ligand (I) (or (I')) is used for catalyzing the synthesis of the asymmetric addition of trimethylsilyl cyanide to aldehydes to give optically active cyanohydrins, enantioselectivity more than 90% is rarely, if ever, observed. Ligands which can be used for catalyzing the synthesis of the asymmetric addition of trimethylsilyl cyanide to aldehydes to give optically active cyanohydrins with enantioselectivity greater than 90% are in high demand now.